International safety standards and a desire to make the safest possible probes for use by oscilloscope operators create a need for a probe design that can tolerate certain types of component failures without subjecting the operators to dangerous voltages. Decreasing probes sizes complicate the attainment of this objective.
Referring to FIGS. 1 & 2, passive voltage probes, such as probe 10 in FIG. 1, have previously been built using an R-C (resistance-capacitance) voltage divider component 20 fabricated using "hybrid" technology. The hybrid R-C component 20 is typically located in a cylindrical probe head housing 12 of the probe 10 and is part of a voltage divider network. In a passive probe the rest of the voltage dividing network is inside the oscilloscope proper, where active circuitry takes over the processing of the signal. One end of the hybrid R-C component 20 is electrically connected by a conductor 13 to the probe tip 14, while the other end is connected by another conductor 15 to circuitry in the oscilloscope (not shown). In a probe with active circuitry, the other end of the hybrid R-C component connects to that circuitry.
The circuit implemented by the hybrid R-C component 20 is shown in FIG. 3. The capacitance of this circuit is implemented as two series capacitors C1 and C2 so that a shorting failure of either capacitor does not lead to a short of the overall component. However, this splitting of the desired capacitance into two series capacitors means that each of the resulting capacitors has to be twice the value of the desired overall capacitance.
The prior art approach to constructing the hybrid R-C component 20 is shown in FIG. 4. The resistance R1 is implemented as resistive element 22 disposed on the bottom side of ceramic substrate 21. A first conductive coating 27 is disposed on the top surface of the ceramic substrate 21. A second conductive coating 25 electrically connects to one end of the resistive element 22 and wraps around to the top side of the ceramic substrate 21. Similarly, a third conductive coating 26 electrically connects to the other end of the resistive element 22 and wraps around to the other end of the top side of the ceramic substrate 21. The first conductive coating 27 is electrically isolated from both second conductive coating 25 and third conductive coating 26.
Dielectric layer 28 covers the first conductive coating 27 and the ceramic substrate 21 in those areas that provide electrical isolation between the three conductive coatings 25, 26, 27. Portions of both the second and third conductive coatings 25 and 26 wrap up and over dielectric layer 28 where they each form one of the plates of capacitors C1 and C2. The first conductive coating 27 also forms capacitor plates for capacitors C1 and C2, since part of it is opposite and parallel to the plate formed on top of dielectric layer 28 by second conductive layer 26 and part of it is opposite and parallel to the plate formed on top of dielectric layer 28 by first conductive layer 25. Thus, capacitor C1 is formed by a portion of first conductive coating 27 and a portion of second conductive coating 25 in conjunction with a portion of dielectric layer 28, and capacitor C2 is formed by a portion of first conductive coating 27 and a portion of third conductive coating 26 in conjunction with a portion of dielectric layer 28.
A bottom moisture barrier 23 covers resistive element 22, while a top moisture barrier 24 covers the exposed portion of dielectric layer 28 and the portions of first and second conductive coatings 25 and 26 that are in contact with the dielectric layer 28 and which form the plates of C1 and C2.
Referring next to FIG. 5, there can be seen a partially cut-away top plan view showing the relationship between the conductive coating layers that form the capacitor plates the solid lines in FIG. 5 show a partial cross-sectional view perpendicular to that shown in FIG. 4 taken at 1--1. The dotted lines in FIG. 5 show a similar partial cross-sectional view taken at 2--2 in FIG. 4.
The first conductive coating 27 includes a first capacitor plate area 30 and second capacitor plate area 31. A first extended area 32 extends from the first capacitor plate area 30 and a second extended area 33 extends from the second capacitor plate area 31. A portion (dotted lines) of the second conductive coating 25 defines a first capacitor plate area parallel to and roughly coextensive with the first capacitor plate area 30 of the first conductive coating 27. Similarly, a portion (dotted lines) of the third conductive coating 26 defines a second capacitor plate area parallel to and roughly coextensive with the second capacitor plate area 31 of the first conductive coating 27. Trim area strips 41 (dotted lines) extend from second conductive coating 25 and partially overlay the first extended area 32 of the first capacitor plate area 30 of the first conductive coating 27. Similarly, trim area strips 42 (dotted lines) extend from third conductive coating 26 and partially overlay the second extended area 33 of the second capacitor plate area 31 of the first conductive coating 27.
There are limits to the voltages that can be applied across the hybrid R-C component 20 that arise from how close together the capacitor plate formed by conductive coating 25 and the capacitor plate formed by conductive coating 26 are to each other. If these plates are kept far apart, the resulting geometries limit the total capacitance that can be produced. If these plates are fabricated too close together, the resulting structure is vulnerable to a high voltage arc over between the edges of the plates. And, since size and weight are significant issues in the design of probes, especially given the long term trend toward increasing miniaturization, an approach that allows for increased electrical safety and increased circuit capacitance is highly desirable.